As this type of optical temperature sensor there is, for example, a known fluorescent temperature sensor wherein a light source, as a light emitting device and a photoreceiving element are spatially separated, as illustrated in U.S. Pat. No. 5,470,155 (“the '155 patent”). In this fluorescent temperature sensor, light from a light source is caused to be incident on an optical fiber that faces a fluorescent material, through the half mirror or a dichroic mirror on the one hand, and the fluorescent light that is emitted from the fluorescent material is caused to illuminate the photoreceiving element through the half mirror or dichroic mirror.